INSTRUCTIONS
1. Preheat an oven to 350°F. Butter an 8-by-11-inch baking dish.
2. In a large saucepan medium-low heat, melt the 8 Tbs. (1 stick) butter. Stir in the flour and cook, whisking constantly, for about 3 minutes. Whisk in the nutmeg. Increase the heat to medium-high and gradually whisk in the milk and stock. Bring the mixture to a boil and continue whisking until thickened and smooth, 10 to 15 minutes. Remove from the heat and let cool, stirring occasionally, until warm. Stir in the Marsala, eggs and 1 cup of the fontina until the cheese melts and the sauce is smooth. Season with salt and white pepper. Set aside.
3. In a large fry pan over medium-high heat, warm 2 Tbs. of the olive oil. Add the leek and sautee until just wilted, about 3 minutes. Transfer to a bowl. Warm the remaining 2 Tbs. olive oil in the pan. When the oil is hot, add the mushrooms and sautee, stirring, until golden, about 5 minutes. Add the prosciutto and basil and cook, stirring, for 1 minute more. Transfer to the bowl with the leek.
4. Spread about 1 ½ cups of the cheese sauce evenly over the bottom of the prepared baking dish. Spoon about one-third of the mushroom mixture evenly over the sauce, then arrange 4 of the lasagna sheets over the top. Repeat these layers twice. Spread the remaining sauce over the top layer, then sprinkle the remaining ¾ cup fontina and the Parmigiano-Reggiano evenly over the top.
5. Transfer to the oven and bake until the top is golden and the juices are bubbling, about 45 minutes. Let the lasagna cool for 15 to 30 minutes, cut into squares and serve immediately.